Texas Geology

Maps

Texas

Ledgend

• The map below is a contour elevation map of a layer of rock that was also as flat as a pancake when it originally formed in the distant geologic past. However, since its formation, that layer has been broken and displaced into huge mountains and basins that are now covered by younger rocks. Indeed, if we could strip away all of the other rocks down to that layer, we would see a landscape with topographic relief similar to what we see today around the highest point in Texas in the Guadalupe Mountains, only greater. There, the Guadalupe Mountains soar 5100 feet above the adjacent valley floor of the Salt Basin. However, in this buried mountainous topography of the Permian Basin the Central Basin Platform looms 7000 feet above the floor of the Delaware Basin.

• The different colors on the map indicate the depth below sea level to that ancient layer which geologists have named "the Wolfcamp formation.". The blue areas are the deepest parts of the basin and the pink areas are the shallowest parts. The deepest blue area is 4500 feet below sea level whereas the highest pink area is 2500 feet above sea level. The blue basin on the left is called the Delaware Basin and the green basin on the right is called the Midland Basin. The pink area between the two basins is called the Central Basin Platform. Together, all three are called-- the Permian Basin. The map was generated by using information from 58,000 wells in the basin.

• The map is contoured on the top of the Wolfcamp formation and is a Regional 3rd Order Residual.  Map courtesy of Geological Data Services.

• Below is a West to East Cross-section of the Permian Basin.  The cross-section gives the impression that a giant knife had cut through the Permian Basin in order to see the rocks in the subsurface.  By combining the above contoured map with the cross-section below, one can observe the Central Basin Platform high in the middle with the deep Delaware Basin to the left and shallower Midland Basin to the right.

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• Geologic Map of Texas• • The geologic history of Texas is recorded in the rock strata that fill the many subsurface sedimentary basins and

crop out across the state. The origin of these strata documents a changing geography that began several billion years ago in the Precambrian Era. Mountains, seas, rivers, volcanoes, and earthquakes are part of the geologic story of Texas, and the resources produced by geologic phenomena (petroleum, coal, lignite, metals, ground water, salt, limestone, ceramic clays, and various soils) are the legacy of the state's changing face.

• Texas is underlain by Precambrian rocks more than 600 million years old. The deformed ancient volcanic and intrusive igneous rocks and sedimentary rocks were formed early in the Earth's history. They are now exposed in the Llano Uplift and in a few small areas in Trans-Pecos Texas.

• During the early Paleozoic, broad inland seas inundated the stable West Texas region (Texas Craton), depositing widespread limestones and shales. Lower Paleozoic rocks are now exposed around the Llano Uplift and in the mountains of Trans-Pecos Texas. The Texas Craton was bordered on the east and south by the Ouachita Trough, a deep-marine basin extending along the Paleozoic continental margin from Arkansas and Oklahoma to Mexico. Sediments accumulated in the Ouachita Trough until late in the Paleozoic Era when the European and African continental plates collided with the North American plate. Convergence of the North and South American plates in this area produced fault-bounded mountainous uplifts (Ouachita Mountains) and small basins filled by shallow inland seas that constituted the West Texas Basin.

• Broad limestone shelves and barrier reefs surrounded the deeper parts of the marine subbasins. Rivers flowed to the landward edges of the basins, forming deltas, and coastlines shifted repeatedly as nearshore sediments were deposited and then eroded by marine processes. Pennsylvanian strata that are products of these processes are exposed today in North-Central Texas. Near the end of the Paleozoic Era, the inland seas retreated southwestward, and West Texas became the site of broad evaporite basins where salt, gypsum, and red muds were deposited in a hot, arid climate. The strata originally deposited in the Permian Basin are exposed in the Rolling Plains of West and Northwest Texas and in Trans-Pecos Texas.

• The Mesozoic Era in Texas began about 245 million years ago when the European and African plates began to break away from the North American plate, producing a belt of elongate rift (fault-bounded) basins that extended from Mexico to Nova Scotia. Sediment from adjacent uplifts was deposited in these basins by streams. While Europe and Africa drifted farther away, the basins were buried beneath marine salt as the East Texas and Gulf Coast Basins were created. During the rest of the Mesozoic Era, broad limestone shelves were periodically buried by coastal plains and deltaic deposits as the Texas continental margin gradually shifted southeastward into the Gulf of Mexico. In the East Texas Basin, deeply buried salt deposits moved upward forming salt ridges and domes, providing a variety of folded structures and traps for oil and gas.

• In West Texas, during the early Mesozoic Era, a large shallow lake occupied the abandoned site of the Permian Basin, but eventually waters from the Gulf of Mexico encroached and flooded West Texas beneath a shallow sea. Dinosaurs roamed the land and shallow waters, and marine reptiles dominated the Mesozoic seas until the waters withdrew from West Texas, near the end of the era. Mesozoic strata are exposed along the western and northern margin of the Gulf Coast and East Texas Basins and extensively across West Texas.

• When the Cenozoic Era dawned in Texas, about 66 million years ago, the East Texas Basin was filling with lignite-bearing deposits of river and delta origin. The early Cenozoic Mississippi River flowed across East Texas, and a large delta occupied the region north of Houston. Smaller deltas and barrier islands extended southwestward into Mexico, very much like the present Texas coast. Delta and river sands were transported southeastward into progressively deeper waters of the Gulf of Mexico. In the Gulf Coast Basin, deeply buried lower Mesozoic salt moved upward to form domes and anticlinal structures. Now, Cenozoic strata are exposed throughout East Texas and in broad belts in the coastal plain that become younger toward the Gulf of Mexico.

• In Trans-Pecos Texas, extensive Cenozoic volcanoes erupted, thick lava flows were deposited over older Mesozoic and Paleozoic strata, and rift basins were formed. Cenozoic volcanic rocks are now well exposed in the arid region of Trans-Pecos Texas.

• In northwestern Texas, late Cenozoic streams deposited gravel and sand transported from the Rocky Mountains of southern Colorado and northern New Mexico. During the Ice Age (Pleistocene Epoch, beginning about 2 million years ago) the Pecos River eroded northward into eastern New Mexico and isolated the alluvial eolian deposits of the Texas High Plains from their Rocky Mountain source. The isolated High Plains were eroded by several Texas rivers during and since the Ice Age, causing the eastern margin (caprock) to retreat westward to its present position.

• While the northern part of the continent was covered by thick Pleistocene ice caps, streams meandered southeastward across a cool, humid Texas carrying great volumes of water to the Gulf of Mexico. Those rivers, the Colorado, Brazos, Red, and Canadian, slowly entrenched their meanders as gradual uplift occurred across Texas during the last 1 million years. Sea-level changes during the Ice Age alternately exposed and inundated the continental shelf. River, delta, and coastal sediments deposited during interglacial (high-sea-level) stages are exposed along the outer 80 kilometers of the coastal plain. Since sea level reached its approximate present position about 3,000 years ago, thin coastal-barrier, lagoon, and delta sediments have been deposited along the Gulf Coast.

• Texas is a composite of nature's processes. Texas today is but one frame in a dynamic geological kaleidoscope of changing rivers, subsiding basins, shifting beaches, uplifting mountains, and eroding plateaus. The face of modern Texas is the link that connects its geologic past to its inevitable future.

• Texas is a microcosm of the American south, plains, Gulf, and Rockies. The Llano Uplift in the center of Texas, exposing ancient rocks of Precambrian age (red), is an outlier of the Appalachian Mountains (along with small ranges in Oklahoma and Arkansas); the Marathon range in west Texas is another. The great exposures of Paleozoic strata shown in blue in north-central Texas were laid down in a shallow sea that retreated westward, ending with the deposition of rocks in the Permian Basin in north and west Texas. Mesozoic strata, covering the middle of the map with their green and blue-green colors, were laid down in another gentle sea that extended from New York to Montana for many millions of years.

• What is The Barnett Shale?• The Barnett shale is named after  early day settler John W. Barnett who homesteaded in San

Saba County Texas where he named a local creek the Barnett Creek.  During the early 20th century during a geological mapping expedition a geologist noticed the black shale outcropping in Barnett Creek and named it the Barnett shale after the creek. In that area of the state the formation outcrops or touches the surface. It is in the seven to eight thousand foot deep range near Dallas.

• What creates gas and condensate in the Barnett shale is organic matter called kerogen that lies embedded in the shale. Through biogenic and thermogenic action, natural gas is given off. The Barnett shale is rich in organic matter and some believe that biogenic action is still producing natural gas even now.  The Barnett Shale is known as a "tight" gas reservoir meaning that the gas is not easily extracted. The formation is a very hard shale and it was virtually impossible to produce gas in commercial quantities from this formation until recent improvements were made in fracturing technology and directional drilling. Large quantities of fresh water are used to frac the Barnett shale. Surface water supplies may be depleted during this process.

• How Old Is The Barnett Shale and How Large Is It?• It consists of Mississippian age (354-323 million years ago) sedimentary shale rocks and

stretches  from Dallas to west  of Fort Worth and southward, covering at least 5,000 square miles in the Fort Worth Basin. The Barnett shale is also found as far West as Pecos County in West Texas. There have been exploratory wells drilled in Reeves County in the Barnett shale with mixed results but exploration continues there. (See map of Barnett Shale below)

• How Much Gas Does The Barnett Shale Contain?• Oil and Gas Experts have suggested that it may be the largest onshore natural gas field in the

US although new estimates from the newly discovered Marcellus Formation in Appalachia may equal the Barnett. The field is proven to have 2.5 trillion cu. ft  of natural gas and the figure could be higher as the boundaries of the Barnett shale are pushed.  Besides natural gas condensate and  light oil  has  been found in lesser quantities and sufficient enough at today's prices to be commercially viable. Below is a chart showing the amount of natural gas production from the Barnett Shale.

• The Haynesville Shale formation is a rich natural gas field underlying northwest Louisiana with extensions into Texas and Arkansas. 

• A few of the more prolific Haynesville Shale Formation wells were drilled in the parishes of Bossier, Caddo, DeSota, &  Natchitoches Parishes of Louisiana, and Harrison County of eastern Texas.  .

• As development wells are completed and leasing activity increases, it is apparent that the size of the Haynesville Shale natural gas field is expanding.  Reserve estimates for the field range from a low of about 30 trillion cubic feet to over 300 trillion cubic feet.  So, this is a very big natural gas strike and the Haynesville natural gas field could become the U.S. largest natural gas field .(The Marcellus Shale natural gas field of the eastern U.S. may challenge that statement....It's growing also!).  It is also conceivable that the natural gas contained in the Haynesville field could constitute one of the largest natural gas fields in the world but that is far from proven.

• In any event, the Haynesville shale field is large and is growing in leaps and bounds.  Some experts are calling this one "The Big One!"

•  • How Far Does the Haynesville Shale Formation Extend?• Although The Louisiana parishes most blessed by the Haynesville Shale strike are located in

northwest Louisiana, it is likely that other parishes will join the action as the gas field is expanded by additional drilling.

• It should be noted that, as the Haynesville Formation extends to the north (toward Arkansas),  the formation appears to become more sandy.  This sand may contain oil and natural gas but is not considered as rich as the predominantly shale formation to the south.  This general statement may also be true for the extensions of the Haynesville Formation into Texas.  (however, the general statement may be refuted by recent well completions in eastern Texas which produced a bonanza of natural gas.)

• The truth is the dimensions of the Haynesville Formation are not known for sure as of now!• A decade or so ago, the oil industry had little use for shale formations.  Now shale is being

eagerly sought!  Both for natural gas and oil (Bakken Shale Formation).